Aromatics production



Feb. 14, 1967 H, D, NQLL AROMATICS PRODUCTION Filed 0G11. 14, 1965 mu www Tu mk Nw N United States Patent Ofi ice 3,304,340 Patented Feb. 14, 1967 3,304,340 AROMATICS PRODUCTION Henry D. Noll, Philadelphia, Pa., assigmor to Air Products and Chemicals, Inc., Philadelphia, Pa., a corporation of Delaware Filed Oct. 14, 1965, Ser. No. 495,932 Claims. (Cl. 260-672) The present invention relates to the production of industrially valuable aromatic and/or alkylaromatic hydrocarbons in -desirably high yield from a hydrocarbon charge stock, particularly from petroleum distillates.

Among the objects of the invention are to produce from a hydrocarbon distillate increased yields of xylenes and benzene, the latter being optionally converted to cyclohexane.

These objects are accomplished, in accordance withthe invention by -reforming a naphtha charge stock of any origin and of suitable lboiling range, and removing from the reformate light parafiins and naphthenes boiling below benzene, the remaining heavier portion of the reformate is split to provide (l) a C6-C8 fraction and (2) a C9+ fraction. Each of these fractions is separately treated under selected conditions to -produce high yields of benzene from the first fraction and to obtain high yields of xylenes from the second fraction. Thus, the first fraction is extracted with a selective solvent to obtain an aromatic concentrate containing benzene and C7C8 alkyl benzenes. This aromatic extract is fractionated to separate an overhead `fraction rich in benzene, a middle cut rich in toluene and a bottoms fraction containing the C8 aromatic hydrocarbons, chiefly xylene. The middle cut (toluene) is subjected to demethylation in the presence of suitable catalyst and the CE-iliquid eluent recycled to the main aromatic fractionator. In this manner the products removed from the main fractionator consist of a benzene fraction and a C2 aromatics fraction consisting chiefly of xylenes.

The C94- fraction separated from the initial reformate is separately demethylated under conditions selected to obtain high yields of C8 aromatics which are recovered from the remaining products in the dealkylation efiiuent, unconverted and heavier (C9-|-) hydrocarbons being continuously recycled to the dealkylation reactor.

The operation of the invention will be understood and the beneficial advantages appreciated from the detailed description which follows read in connection with the accompanying drawing illustrating by simplified schematic iiow diagram a preferred embodiment thereof. Conventional auxiliary equipment, such as heat exchangers, compressors, reboilers, pressure reduction valves, circulating pum-ps, etc. have largely been omitted from the drawings for clarity in presentation.

There is shown in the flow diagram at 10 a hydrogenative refining system wherein the naphtha charge is contacted with hydrogen and passed over catalyst under conditions designed to reduce any sulfur and/or nitrogen compounds present. Of course, if the selected charge stock is relatively free of sulfur and nitrogenous contaminants the hydrorefining treatment need not be practiced. However, if the hydrocarbon charge contains as much as 0.02% sulfur compounds it is desirable that these be removed, since the presence of sulfur compounds in the feed to the subsequent reforming operation reduces the yield of aromatics obtained and also reduces catalyst life.

The refined product from 10 is sent'to a gas separator at 11 where the H2S land H2 together with light hydrocarbon gases are removed overhead by line 12 andthe remaining liquid hydrocarbons passed to the reforming system through line 13. The hydrogen-containing gas from 11 may be recycled to the refining step at 10, if desired after treatment by scrubbing in known manners to remove H2S and/or ammonia compounds therein, or a portion of the gas may be discharged and the remainder recycled to the treating reactor at 10, without purification, making up with the fresh hydrogen supplied an equilibrium hydrogenrich stream of the required degree of purity. Y

The :reforming section 14 is similar to that generally employed in hydrogen-ative reforming of naphtha over platinum catalyst, comprising generally three or more reactors in series. In typical reforming of hydrocarbons over platinum catalyst a number of simultaneous reactions generally take place, including: dehydrogenation of naphthenes, d-ehydrocyclization of paraffins, hydrocracking of paraiiins, and isomerization of paraflins and part of the naphthenes. By proper selection of catalyst and operating conditions, the comparative rates of these various reactions can be controlled to some extent. In the present instance the catalyst and operating conditions are selected to favor increased aromatics production.

The effluent from the reforming operation at 14 is flashed at 15 to remove hydrogen and lower gaseous hydrocarbons, withdrawn by line 16, part of which hydrogen-rich gas is recycled to the reforming reaction. The remainder of the gas is used in the pretreating section 10 and elsewhere in the system in which hydrogen is required.

The liquid product Ifr-om 15 is sent by line 17 to a stabilizer 18 and splitter 19 to effect removal of any remaining hydrogen and light hydrocarbons such as those boiling below benzene. C6 parafiins and lighter products are thus separated in the overhead streams 20 and 21 :from the higher yboiling liquid hydrocarbons withdrawn through line 22. The products in line 22 will thus include principally benzene and higher boiling cyclic and acyclic hydrocarbons. Splitter 19 may be operated at about atmospheric pressure and at a cut point of about -78 C. to obtain substantially all of the benzene and higher boiling hydrocarbons in the bottom product.

The products in line 22 are fractionated at 23 4at a suitable cut point to separate an overhead fraction composed chiefly or almost entirely of C-CB hydrocarbons, and a bottoms fraction composed of C9 and heavier hydrocarbons. The cut point for this fractionation may be taken within the C9 paran Iboiling range or at about 140-150 C. so that substantially all of the trilmethyl benzene cornpounds remain in the bottoms fraction while substantially v tion such separation is accomplished by extraction with a selective solvent for the aromatic components such as: N-methyl pyrrolidone, N-hydroxyethyl pyrrolidone, butyrol-actone, triethyleneglycol, dimethyl formamide. These solvents are diluted with a small amount of water or other anti-solvent in known manner to improve selectivity. The obtained aromatic concentrate, after (being freed of extraction solvent, is sent by line 26 t-o the main aromatics fractionator 27.

The C9+ hydrocarbons withdrawn from fractionator 23 through line 28 are subjected to hydrogenative dealkylation at 29, conditions being selected to favor production of xylene compounds. Any non-aromatic compounds presen-t are cracked to acyclic 4hydrocarbons boiling below benzene. The effluent reaction products from dealkylation at 29 are stripped of hydrogen and low boiling hydrocarbons (through C5) in stabilizer 30 and remaining liquid products sent by line 31 to fractionator 32 wherein separation -is made between C8 and lighter products taken overhead through line 33 and C9| hydrocarbons recycled to dealkylation in reaction system 29 by means of line 34.

The charge to the main aromatics fractionator 27, thus comprises varomatics concentrate essentially free of acyclic compounds (C5-C3) :from the aromatic extraction systems 25 and the product from line 33 which is rich in C8 aromatics but which also contains accompanying benzene and toluene formed by dealkylation in 29. The fractionator 27 is operated to separate the charge into an overhead fraction consisting almost entirely of benzene of high purity (nitration grade), a heart cut consisting largely of toluene and a bottoms cut composed of C8 aromatics. The benzene fraction is withdrawn through line 35 and can be sent to storage as a product of marketable purity. If desired, however, all or part of the benzene can be hydrogenated to more valuable cyclohexane as indicated at 36. The C8 aromatics cut is Withdrawn through line 37 and sent to storage 38. The stored product comprises a mixture of the Xylene isomers with some ethyl benzene.

The toluene cut is withdrawn at an intermediate level. of fractionator 27 through line 39 and subjected to demcthylation as indicated at 40. The reaction product from demethylation is stripped of hydrogen and hydrocarbons boiling below benzene in stabilizer 41 while the higher boiling liquid hydrocarbons (C6 and C7), composed of product benzene and unreacted toluene are returned to the main aromatics fractionator 27 through line 42. In the main fractionator these returned products will be separated into :benzene and toluene, the former being collected with the benzene in line 3S while the latter is again sent to demethylation in 40 to form additional benzene.

It will thus be seen that in the particular arrangement provided there is obtained lfrom a suitable boiling range naphtha, aromatic products having high industrial demand and enhanced market value consisting of a xylene fraction which can be marketed as such or split into its several isomers, and a benzene fraction which in itself is in demand and which optionally can be readily converted in high yields to presently more valuable cyclohexane. The toluene cut, which is less profitable, is continuously recycled to extinction with production of ad-ditional benzene. By separately demethylating the C9-lfraction and the C, fraction, conditions in each reaction are more readily controlled to obtain maximum yields of 'xylene and benzene respectively with only small amounts of the C9 charge being dealkylated -beyond the dimethyl benzene stage, and even this small amount being readily recovered as benzene or by conversion to benzene. By provision of the single main fractionator for all of the aromatics products, moreover, consi-derable saving in equipment and operation costs are had.

The operating conditions for each of the reactions will now be described.

Desulfurzalion of charge As explained above, this step need be practiced only if the naphtha employed contains more than 0.02% S and/ or more than 100 ppm. N. The pretreatment is carried out in a manner and under conditions generally employed in connection with pretreatment of a nap'htha charge to be subjected to hydrogenative reforming over supported platinum catalyst. In the typical catalytic hydro-treatment for removal of sulfur and nitrogen compounds, the naphtha charge is passed over cobalt molybdate catalyst at 650- 750 F. and at pressure of about 400-750 p.s.i.g. at hourly liquid volume space rate of l to 4 and at a hydrogen to oil ratio of l to 3. For charge stocks containing nitrogen as well as sulfur conta-minants pressure at the higher level of the indicated range is preferred and space rates in the lower part of the stated range. A typical commercial catalyst that can be employed for this purpose is described in U.S. Patent No. 2,755,257. By the described hydrogenative treatment, the sulfur and nitrogen contaminants in the charge stock lare converted largely to HZS and NH3 respectively which, together with hydrogen and light hydrocarbons (through C4 or C5) are stripped out at 11 from the remaining liquid products sent to reforming. If desired, hydrogen-rich gas is recyled to the hydrotreating reaction, a portion being removed and replaced by hydrogen from line 16 to maintain desired hydrogen concentration and purity.

REFORMING SECTION is more usually of the Igamma or eta type; in some -instances the catalyst is used in sulded state. These catalysts differ in acidity as a result of difference in content of halogen o-r other properties of the alumina carrier. While most of the available commercial platinum-alumina catalysts can be employed for the presently described reforming step, to obtain selectively high yields of desired aromatic compounds without excessive hydrocracking and isomerization, it is preferred to employ platinum catalysts of low acidity value. Such catalysts are generally free from fluoride and contain halogen, if any, only in the form of chloride in an amount less than the platinum content of the catalyst. A recommended catalyst for this purpose is that described in U.S. Patent No. 3,189,559. In reforming operations over platinum catalyst a wide range of operating conditions have heretofore been suggested including pressures of 100 to 1000 pounds per square inch, temperatures of 750 to 1050D F. and space rates of 0.1 to 5.0 with hydrogen to oil ratios of 1/ l to l0/1 or more. To favor aromatics production, in addition to using the low acidity platinum alumina catalyst heretofore described, it is advantageous to observe operating conditions within a more narrow range. The temperature should be at about 930 F. i20 F when starting on fresh active catalyst and should be increased -progressively to about l000 F. as the catalyst becomes less active over the period of onstream use until regeneration is effected to res-tore the catalyst activity. Space rates of .about 1 to 3 volumes of oil per hour per volume of catalyst, preferably at about 2, and pressures of about 300 to 500 p.s.i.g. favor aromatics production particularly when using low acidity platinum-alumina catalyst. Hydrogen-rich gas should be recycled at a rate to provide Hz/oil ratios of about 5 or more, preferably about 6 moles/mole of hydrocarbon feed. Under these conditions continuous operation of several months -or more land at times up to about a year is made possible before it is needed to regenerate the catalyst to restore activity.

In the reforming operation, hydrogen is produced as a result of cyclizatifon and dehydrogenation of hydrocarbons. A portion of the produced hydrogen is recycled in the desired quantity to the reforming operation, while the excess hydrogen is sent to the hydro-pretreating step and elsewhere in the system as required.

AROMATICS EXTRACTION The C6-C8 fraction of the reformate (line 24) will contain over 50%-70% or more aromatics boiling in the range from benzene to o-xylene (about -145 C.) in addition to acyclic compounds, chiefly plaratlins, in the CB-Cs boiling range. In the preferred embodiment separation of the aromatics is achieved by extracting the hydrocarbon Icharge in multistage counterow apparatus with: about 3 to 4 volumes of N-methyl pyrrolidone containing 15% Water and 10 l5% pentane. The solvent is separated from the aromatic hydrocarbons by distillation and reused for further extraction.

DEMETHYLATION OF TOLUENE While the demethylation of toluene can be accomplishedv thermally, in such operation temperatures .above about 1225 F. are required with accompanying excessive degradation of product. It is therefore preferred to employ catalytic demethylation over highly active chrome-alumina catalyst containing 15-25 Cr2O3 supported on an alumina base obtained by dehydration of an alumina hydrate containing at least 50% beta alumina ytrihydrate. Conditions used for this operation are described in U.S. Patent No. 3,178,486. In general, the demethylation should be carried out at about 1000-1200" F., pressures of 650-800 pounds per square inch, and rates giving contact time of 50-150 seconds. The preferred operation in accordance with one specific embodiment is carried out at a pressure of 700 pounds per square inch, maintaining a temperature of 1150 F. (at the reactor outlet) at a hydrogen/oil mole ratio of 5 (7:1 total recycle gas to oil) and at a space rate of about 0.45, resulting in contact time of about 70 seconds. The catalyst employed is that described in Example I of the aforesaid U.S. Patent No. 3,178,486, having an activity corresponding to a kT value (at 873 K.) of approximately 0.602 as compared to kT=0.l44 for the non-catalytic thermal dealkylation at this temperature.

DEALKYLATION OF C9 AROMATICS The hydrocarbon charge subjected to dealkylation in system 29 will contain typically Percent C9 and C10 parains 3-6 C9 aromatics 60-70 C10 aromatics 20-30 C11 aromatics Circa 2 Indene Iand naphthalene, less than 0.5

By 'subjecting this charge to dealkylation conditions over chrome-alumina catalyst, the small amount of C9 and C10 parains in the charge will be all cracked out .chiey to low molecular weight (C5 and lower) gases. The `alkyl aromatics will largely demethylate stepwise obtaining largely xylene isomers in the liquid product with some formation of benzene and toluene.

Conditions of reaction favoring production of xylene with minimized further demethylation to toluene and benzene are the use of eq-ual or somewhat higher pressures and lower temperatures and shorter contact time than those employed in the demethylation of toluene. For xylene production from C9 aromatics, temperatures should be in the order of 1100 to 1150 F., pressures of 750 to 900 p.s.i.g. and contact times generally below 1 minute, which entail space rate (LHSV) approaching or somewhat exceeding 1 (vol./hr./vol.). In a preferred specific embodiment dealkylation of the C9 aromatics fraction is effected at about 1l30 F. (reactor outlet temperature),

pressure of 800 p.s.i.g., at 7:1 moles recycled gas t-o oil (corresponding to about 5/1 Hzroil mole ratio) and at a liquid hourly space velocity of 1.2 (vol./hr./vol.), affording a contact time of about 40 seconds.

Example The S55-200 C. virgin naphtha fraction derived from a Kuwait crude, analyzed as follows:

Percent by wt. Parafns 62.0 Naphthenes 24.0 Aromatics 14.0 Sulfur total about 0.03

The above-described naphtha fraction is desulfurized in a hydro-pretreater over cobalt-molybdate catalyst, resulting in a produ-ct having less than 50 p.^p.'m. sulfur, and essentially free of nitrogen. The desulfurization is effected at 700-750 F., 400 p.s.i.g., 'at a liquid hourly space rate of 4 (vo1./.hr./vol.) and a recycle gas mole ratio of 2.5.

The products `from the hydro-pretreater are stripped of hydrogen, H2S, and light gases and the remaining C54- liquid sent to hydrogenative reforming over platinumalumina -catalyst in a three-reactor fixed bed system with intermediate reheaters. The lreforming is effected at an average temperature of 935 F., 380 p.s.i.g. lat the inlet to the first reactor, tat an hourly volume space rate of 2 and at a gas recycle ratio 8 moles gas per mole of liquid 6 hydrocarbon feed. The resulting reformate product has the following composition:

Wt. percent H1 and C1 to C4 paraflins 22.6 C5 paraffins 10.0 C6 parains 8.7 C6 and C7 naphthenes 0.6 Benzene 2.4 C7 parafiins 6.9 Toluene 9.8 C8 paraffins 2.7 C8 aromati-cs 13.9 C9 arom-atics 13.9 C10 4aromatics 5.5 Olens 1.3 Other hydrocarbons 1.7

The reformate is cooled and flashed a-t the existing pressure to -remove hydrogen rich gas for recycle to the reforming section and for other uses. The degassed product is futher treated to separate out .product-s lboiling below benzene and the remaining liquid product (C6 aromatics to 200 C.) is fractionated to provide a C6 to C8 hydrocarbon fraction sent to aromatics extraction and a Cg-lhydrocarbon fraction sent to dealkylation. Of the original 8.7% parafns in the reformate, 76% thereof is removed in the stabilizer overhead, the remainder appearing in the benzene and heavier liquid fraction sent to aromatics extraction.

The compoistion of the C6 to C8 fraction is as follows, the aromatics being substantially recovered in full by the extraction.

Feed to Extraction Extract Weight Weight Percent Percent Benzene 6. 2 9.8 Toluene 25. 2 39.8 Cs aromatics 33. 5 50, 4 Non-aromatics 35. 1

Percent C8 aromatics 4 C9 aromatics 64 C10 aromatics 25 C11 :aromatics 2 Parains and -other hydrocarbons 5 Said feedstock is directed over a catalyst consisting of 20% Cr2O3 on 80% eta alumina at a pressure of 800 p.s.i.g and a contact time of about 46 seconds at about 5 to 1 hydrogen to oil ratio (7 to 1 gas to -oil recycle ratio) at an average reaction temperature of about 1130" F. (1040 F. inlet, 1135J F. outlet) at a liquid hourly space velocity of about 1.05 volumes of oil per hour per volume of catalyst. Particular attention is directed to the feature of utilizing the combination of low reaction temperature and short contact time, whereby the dealkylation of the C9 hydrocarbons favors formation of xylene with a smaller amount of toluene and a still smaller amount of benzene. The reaction conditions employed result in about 70-80% conversion of the C9-lhydrocarbons charged to conversion; 50-70% by volume of the liquid product is lcomposed of C8 aromatics. The remaining 2030% of unconverted and partially converted product is recycled to dealkylation.

The dealkylated product flows through line 33 for adrnixture with the aromatics from the extraction zone 25, tand the mixture flows to the fractionator 27.

The C7 aromatics stream in line 39 is demethylated over the chrome-alumina catalyst above described in a sys-tem comprising two reactors in series at an inlet temperature of 1090 F. `to the rst reactor and at 1130o F. to the second reactor, at a pressure of 700 p.s.i.g., at a total hourly space rate f 0.4 (v-oL/hn/ vol.) and at a 5:1 mole H2/oil ratio. These conditions obtain a residence time of about 70 `seconds in the two reactors and result in about 70% conversion Iof the toluene charged. Unconverted toluene is recycled to extinction.

Obviously, many modifications and variations of the invention :as hereinbefore set forth may be made Without departing from the spirit and scope thereof, and therefore only such limitations should be imposed as are indicated in the appended claims.

What is claimed is:

1. In the method of producing benzene and Xylenes from a hydrocarbon reformate rich in aromatics the improvement which comprises: removing hydrocarbons boiling below about 7S-78 C. from said reformate; fractionating the remaining hydrocarbons after such separation into (a) a light fraction composed predominantly of C6 to C8 hydrocarbons, and (b) a heavy fraction consisting essentially of C9 and higher hydrocarbons; extracting aromatic hydrocarbons from said light fraction (21); subjecting said heavy fraction (b) to selected conditions of mild hydrogenative dealkylation at` temperatures between about 1100 and about 1150 F. over active chrome-alumina catalyst favoring production of Xylenes; separating C6 to C8 hydrocarbons from said mild dealkylation while continuously recycling to said dealkylation products boiling above Xylenes; combining the separated C6 to C8 hydrocarbons stream with said aromatic extract stream from the light fraction and subjecting the combined streams to fractional distillation to provide: (c) an overhead cut consisting substantially of benzene; (d) a bottoms cut composed essentially of CB aromatic hydrocarbons predominating in Xylenes; and (e) a middle cut consisting essentially of toluene; subjecting said middle cut (e) to severe demethylation over active chrome-alumina catalyst at temperatures between about 1000 and about l200 F. under selected conditions favoring production of benzene; removing products boiling below benzene from the effluent from said severe demethylation reaction, while returning the produced benzene and unreacted toluene to fractional distillation with the aforesaid combined streams so as to obtain (l) additional recovered benzene products and (2) toluene for recycling to demethylation; and controlling the mild de- -alkylation of the C9 plus hydrocarbons fraction to provide shorter residence time than that employed in the severe demethylation of the toluene.

2. In the method in which a hydrocarbon fraction is converted to mononuclear aromatic hydrocarbons by steps comprising hydrogenative reforming of a naphtha fraction, extractive distillation of at least a portion of the reforrnate, and hydrogenative catalytic dealkylation of at least a fraction of the alkylated aromatics, the improvement which consists of: subjecting a mixture of hydrogen and feedstock to high pressure catalytic hydrodesulfurization; hydrogenatively reforming the hydrodesulfurized feedstock over a platinum on alumina catalyst; separating and withdrawing from the reformate the liquid boiling below about 75 C. and the liquid boiling above about 200 C. to provide a C6 to C11 fraction boiling in the 80 to 200 C` range; splitting the C6 to C11 fraction to provide a C6 to C8 fraction and a C9 to C11 fraction, said splitting being at a cut point of in the range from 140 to 150 C.; extracting the C6 to C8 fraction to provide a nonaromatic rainate by product and a C6 to C8 aromatic fraction; exhaustively converting the C9 to C11 fraction to C6 to C8 aromatic hydrocarbons by exhaustive recycling through a mild hydrodealkylation zone containing chromia on alumina catalyst, the Contact time being less than about 60 seconds, there being several mols of hydrogen per mol of normally liquid hydrocarbons, and the temperature being in the'range from about 1100 F. to about 1l50 F., whereby any nonaromatic components are converted either to C6 to C8 aromatic hydrocarbons or to by products comprising nonaromatic hydrocarbons boiling significantly below benzene; admixing the C6 to C8 aromatic fractions from the extraction with the C6 to C8 aromatic hydrocarbons from the mild dealkylation steps and fractionally distilling to provide a benzene fraction, a Xylene fraction, and a toluene fraction; exhaustivelyrecycling the toluene through a severe demethylation zone for total conversion to benzene, the recycle passing through said fractional distillation of C-G to C3 aromatic fractions, the hydrodealkylation zone containing chromia on alumina catalyst, the contact time being in the range from about 50 seconds to about 150 seconds and longer than the contact time in the mild dealkylation reaction, there being several mols of hydrogen per mol of normally liquid hydrocarbon, and the temperature being higher than in the mild dealkylation reaction but less than about 1200 F.; and transferring hydrogen generated in the reforming zone for consumption in each of the zones for hydrodesulfurization, mild hydrodealkylation, and severe hydrodemethylation.

3. A method in accordance with claim 2 in which the feedstock to the hydrogenative reforming zone has an end boiling point providing a reformate containing only trace amounts of Cu-lhydrocarbons, whereby substantially only C6 to C10 hydrocarbons are converted selectively to Xylenes and benzene.

4. A method in accordance with claim 2 in which hydrogen generated in the reforming zone is employed in con'- verting at least a porti-on of the benzene to cyclohexane.

5. A method in accordance with claim 1 in which the extraction of aromatic hydrocarbons from the light fraction composed predominantly of C6 'to C3 hydrocarbons is conducted using a solvent consisting of about 15% water, about 15% pentane and about 70% by weight methyl pyrrolidone, there being from about 3 to about 4 vol` umes of solvent per volume of C6 to C8 hydrocarbon.

6. A method in accordance with claim 1 in which the mild dealkylation of C9 plus hydrocarbons is conducted at about 800 p.s.i.g. pressure at about 1l30 F. for acontact time of about 40 seconds.

7. A method in accordance with claim 1 in which the severe demethylation of the toluene fraction is conducted at a temperature of about 1150 F. at a pressure of about 700 p.s.i.g. for a contact time of about 70 seconds.

8. A method in accordance with claim 1 in which the fractional distillation of the C6 to C3 aromatics is conducted to permit benzene, Xylene, and other materials to contaminate the toluene fraction, but to achieve a benzene fraction particularly free from 'toluene and a Xylene fraction particularly free from toluene.

9. A method in accordance with claim 1 in which the mild dealkylation and severe demethylation reactions are conducted in the presence of a recycle gas to normally liquid hydrocarbons of about 7 to 1, and the hydrogen purity is sucient to maintain a hydrogen to normally liquid hydrocarbon ratio of at least about 5 to l.

10. A method in accordance with claim 1 wherein product benzene is hydrogenated to obtain cycloheXane.

References Cited by the Examiner UNITED STATES PATENTS 2,737,538 3/1956 Nelson 260-674 2,780,661 2/ 1957 Hemminger et al. 208-66 2,889,263 6/1959 Hemminger et al. 208-66 2,933,448 4/ 1960 Morin et al. 260--674 3,169,151 2/1965 Merryeld et al. 260-672 3,178,486 4/1965 Maerker et al 260-672 DELBERT E. GANTZ, Primary Examiner.

G. E. SCHMITKONS, Assistant Examiner. 

1. IN THE METHOD OF PRODUCING BENZENE AND EXYLENES FROM A HYDROCARBON REFORMATE RICH IN AROMATICS THE IMPROVEMENT WITH COMPRISES: REMOVING HYDROCARBONS BOILING BELOW ABOUT 75-78*C. FROM SAID REFORMATE; FRACTIONATING THE REMAINING HYDROCARBNS AFTER SUCH SEPARATION INTO (A) A LIGHT FRACTION COMPOSED PREDOMINANTLY OF C6 TO C8 HYDROCARBONS, AND (B) A HEAVY FRACTION CONSISTING ESSENTIALLY OF C9 AND HIGHER HYDROCARBONS; EXTRACTING AROMATIC HYDROCARBONS FROM SAID LIGHT FRACTION (A); SUBJECTING SAID HEAVY FRACTION (B) TO SELECTED CONDITIONS OF MILD HYDROGENATIVE DEALKYLATION AT TEMPERATURES BETWEEN ABOUT 1100 AND ABOUT 1150*F. OVER ACTIVE CHROME-ALUMINA CATALYST FAVORING PRODUCTION OF EXYLENES; SEPARATING C6 TO C8 HYDROCARBONS FROM SAID MILD DEALKYLATION WHILE CONTINUOUSLY RECYCLING TO SAID DEALKYLATION PRODUCTS BOILING ABOVE EXYLENES; COMBINING THE SEPARATED C6 TO C8 HYDROCARBONS STREAM WITH SSAID AROMATIC EXTRACT STREAM FROM THE LIGHT FRACTION AND SUBJECTING THE COMBINED STREAMS OF FRACTIONAL DISTILLATION TO PROVIDE: (C) AN OVERHEAD CUT CONSISTING SUBSTANTIALLY OF BENZENE; (D) A BOTTOMS CUT COMPOSED ESSENTIALLY OF C8 AROMATIC HYDROCARBONS PREDOMINATING IN EXYLENES; AND (E) A MIDDLE CUT CONSISTING ESSENTIALLY OF TOLUENE; SUBJECTING SAID MIDDLE CUT (E) TO SEVERE DEMETHYLATION OVER ACTIVE CHROME-ALUMINA CATALYST AT TEMPERATURES BETWEEN ABOUT 1000 AND ABOUT 1200*F. UNDER SELECTED CONDITIONS FAVORING PRODUCTION OF BENZENE; REMOVING PRODUCTS BOILING BELOW BENZENE FROM THE EFFLUENT FROM SAID SEVERE DEMETHYLATION REACTION, WHILE RETURNING THE PRODUCTED BENZENE AND UNREACTED TOLUENE TO FRACTIONAL DISTILLATION WITH THE AFORESAID COMBINED STREAMS SO AS TO OBTAIN (1) ADDITIONAL RECOVERED BENZENE PRODUCTS AND (2) TOLUENE FOR RECYCLING TO DEMETHYLATION; AND CONTROLLING THE MILD DEALKYLATION OF THE C9 PLUS HYDROCARBONS FRACTION TO PROVIDE SHORTER RESIDENCE TIME THAN THAT EMPLOYED IN THE SEVERE DEMETHYLATION OF THE TOLUENE. 